(a) Field of the Invention
The present invention relates to a method and a system for measuring frequency or frequency deviation of a periodic signal in the range of approximately from 0 to 2 KHz and wherein the measuring is effected by sampling the periodic signal at precise time intervals and providing digital value signals representative of the sampled values for processing to calculate the lapse time of the signal since its previous crossing of the same level.
(b) Description of Prior Art
With the growth of power systems, permissible frequency deviations have been reducing while at the same time frequency-bias, (expressed as megawatts per 0.1 Hz deviation) have been increasing.
The advent of frequency sensitive relays, load behaviour modelling and other computer controlled loops have necessitated measurements of power system frequency with a degree of speed, precision and stability, not provided by conventional electromechanical instruments.
Various methods have been suggested based on measurement of the time between zero crossings of the signals. Others have suggested the calculation of the frequency on the basis of a comparison with a standard and precise periodic signal. Various types of circuitry are employed in these methods such as digital, analog and hybrid circuits.
All the methods stress the importance of an analog meter display even if some have the frequency available in numerical form. All assume signals to be reasonably free from random and high frequency noise so that a zero crossing detector (inherent in all the above methods) will perform satisfactorily. None have any estimate of frequency before half a cycle, and some have longer lags due to the use of filters.
Our studies of the voltage and current waveforms of power systems at bulk power substation level in normal operation show that the random noise present can cause the zero crossing detection, of the prior art, to be quite inaccurate, particularly in view of the small magnitude of frequency variations to be measured.